Making up and breaking out of a tubing string in a well white maintaining continuous circulation

ABSTRACT

An apparatus for making up and breaking out of a tubing string in a well includes a connector ( 9; 109 ) for connection to a tubular ( 1 ) and a main high pressure conduit ( 13; 113 ) communicating with the connector ( 9; 109 ) for allowing circulation through a tubular ( 1 ) connected thereto. Below that unit a connecting shell ( 17; 117 ) bounding a connecting chamber ( 18 ) has an upper passage ( 19 ) and a lower passage ( 20 ), a preventer ( 23 ) for separating an upper portion ( 24 ) of the connecting chamber ( 18 ) from a lower portion ( 25 ) thereof, and a back-up high pressure conduit ( 26 ) communicating with the connecting chamber ( 18 ). A pressure corresponding to the pressure in the upper portion ( 24 ) of the connecting chamber ( 18 ) is provided in at least one pressure chamber ( 51; 151 ) and exerts a force pressing a tubular ( 1 ) or the connector ( 9; 109 ) into the connecting chamber ( 18 ) against forces exerted by pressure in the connecting chamber ( 18 ).

TECHNICAL FIELD

The invention relates to the making and breaking of tubing strings, suchas drill strings and casing strings, suspended in a well drilled orbeing drilled in the lithosphere while continuous circulation of a fluidsuch as mud or foam is maintained.

BACKGROUND ART

An apparatus for connecting and disconnecting tubulars and a tubingstring suspended in a well and for axially displacing that tubingstring, and methods of assembling and disassembling a tubing stringprojecting into a well are known from U.S. Pat. No. 3,559,739. In thisdocument a method and an apparatus for providing continuous foamcirculation in wells is disclosed. To enable the circulation to continuewhile a tubular is connected to the tubing string or disconnected fromthe tubing string, a shell is provided enclosing a space where theconnection is to be made or broken. When the upper end of the tubingstring is opened, the flow via the top drive is taken over by a flowwhich enters or leaves the tubing string via the back-up conduit and theconnecting chamber formed by the shell in which the connection is beingmade up or broken out. To allow the entry of a tubular to be added intothe connecting chamber or the removal of a tubular from the connectingchamber, while maintaining the flow through the open end of the tubingstring, the connecting chamber can be divided in two portions by apreventer. Thus, the flow can be maintained via the high pressureback-up conduit and the lower portion of the connecting chamber while atubular is brought into a position in-line with the tubing string or istransported away from above the tubing string.

Other examples of such a method and such an apparatus are disclosed inInternational Patent Application WO 98/16716, which corresponds to U.S.Pat. No. 6,315,051, which issued to Ayling on Nov. 13, 2001 and which isincorporated herein by reference.

A problem of these methods and apparatuses is to accurately control theaxial movement of and the axial forces exerted by a tubular to beconnected to or being disconnected from the tubing string. Especiallythe stabbing forces immediately before making up a coupling and theretraction forces during the completion of breaking of a coupling aredifficult to control. Inaccuracies in the control of stabbing forces caneasily lead to damage to the coupling members, for instance to thethreads. Apart from the costs of repair or replacement, this alsoentails the risk of coupling failure after the coupling has entered thewell, with the associated hazards and extra operating costs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus and amethod with which control over stabbing and retraction forces is lessimpeded by the pressure in the connecting chamber and which allows tobuild up and remove the pressure in the connecting chamber quicklywithout loosing control over stabbing and retraction forces.

According to one aspect of the present invention, this object isachieved by providing an apparatus of the above-identified type having apressure compensating structure for compensating axial force exerted bypressure in an upper portion of a connecting chamber pressing a tubularprojecting from the upper portion of the connecting chamber in an axialdirection out of the connecting chamber. The pressure compensatingstructure includes at least one pressure chamber and at least onepressure transfer member formed by the connector or by a separatepressure transfer member connectable to a tubular. The pressure chamberhas a passage and means for sealing off the passage against the at leastone pressure transfer member, the at least one pressure chamber isconnected to the connecting chamber for maintaining a pressure in the atleast one pressure chamber corresponding to pressure in at least theupper portion of the connecting chamber. The at least one pressuretransfer member is displaceable in the at least one pressure chamber andis arranged for transferring a force axially, by pressing the tubularinto the connecting chamber in reaction to pressure in the at least onepressure chamber. According to other aspects of the present invention,this object is achieved by, after lowering the tubular into the upperone of the passages, maintaining a pressure in at least one pressurechamber at a level corresponding to pressure in the upper portion of theconnecting chamber, axially urging the tubular towards the connectingchamber and at least partially balancing out upward pressure exerted tothe tubular by the pressure applied to the upper portion of theconnecting chamber. Alternatively, according to other aspects of thepresent invention, this object is achieved by, after lowering the atleast one connector into the connecting shell, maintaining pressure inat least one pressure chamber at a level corresponding to pressure inthe upper portion of the connecting chamber, axially urging theconnector towards the connecting chamber and at least partiallybalancing out upward pressure exerted to the connector by the pressureapplied to the upper portion of the connecting chamber.

It has been found that the stabbing and retraction forces are difficultto control because the pressure in the upper portion of the chamber inthe connecting shell—which can be in the order of magnitude of 500bar—pushes the tubular to be connected or being disconnected out of theconnecting chamber with a force which is substantially larger than thestabbing and retraction forces and which varies with fluctuations of thepressure in the chamber of the connecting shell which occur inoperation.

The present invention provides that the tubular is pushed towards theconnecting chamber by the pressure in the pressure chamber with a forcewhich is at least essentially proportional with the force with which thetubular or the connector is pushed out of the connecting chamber by thepressure in the upper portion of the connecting chamber. Thus, the otheraxial forces which have to be transferred to the connector or thetubular to control the retraction or stabbing forces are substantiallyreduced, substantially more constant and require no or substantiallyless compensation for the forces generated by pressure in the connectingchamber. The devices for controlling and generating the stabbing andretraction forces and movements are substantially relieved from havingto compensate the axial forces generated by the pressure in the chamberin the connecting shell. Therefore, the retraction and stabbing forcesare better controllable and/or can be controlled with less powerfuldrives. Moreover, the pressure in the connecting chamber can be built upand removed quickly and even during stabbing or completion of breaking acoupling, without substantially disturbing the control of the stabbingor retraction forces and displacements of the tubular.

Particularly advantageous modes of carrying out the invention are setforth in the dependent claims.

Further details, objects, features and advantages of the invention aredescribed with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view in cross section of an upper portion ofa first example of an apparatus according to the invention;

FIGS. 2A-2M are schematic side views of an apparatus according to theinvention during successive stages of operation; and

FIG. 3 is a schematic side view in cross section of an upper portion ofa second example an apparatus according to the invention.

MODES FOR CARRYING OUT THE INVENTION

In FIG. 1. an example of an apparatus according to the invention isshown in a stage of operation in which a tubular 1 is being stabbed intothe top end of a tube string 2. This situation essentially correspondsto the stage of operation shown in FIG. 2K.

The apparatus has a suspension structure which can be of an essentiallyconventional construction and includes a vertical guide 5 for guidingthe top end connecting unit 4, a table 6 carrying a clamp 7 which isadapted for taking up axial and rotational loads and a table 58 carryinga second clamp 27 for taking up axial and rotational loads. Theconnecting unit 4 and the tables 6, 58 are vertically movable along theguide 5 as is schematically represented by rollers 59. The top endconnecting unit 4 is movable up and down by cylinders 8. The table 6 ismovable relative to the table 58 by cylinders 60.

The top end connecting unit 4 includes a connector 9 for retaining thetubular 1 in a position axially projecting from the connector 9. To thisend, the lower end 10 of the connector is formed as the lower end 11 ofa male connecting member 12 of a tube coupling. The top end connectingunit 4 further includes a main high pressure conduit 13 of which aportion is formed by a high pressure hose 14 and is connected to apassage 15 through the connector 9 for allowing circulation through thetubular 1 (or any other tubular) connected to the connector 9. At theend of the main high pressure conduit 13 a seal 16 is provided forsealing off the main high pressure conduit 13.

Below the top end connecting unit 4, a connecting shell 17 is located.The connecting shell 17 bounds a connecting chamber 18 and has an upperpassage and a lower passage 19, 20 on diametrically opposite sides forreceiving tubulars. The openings 19, 20 are provided with seals 21, 22for sealing off the passages 19, 20 against tubulars extending throughthe passages. In the connecting chamber 18 a preventer 22 for separatingan upper portion 24 of the connecting chamber 18 from a lower portion 25of the connecting chamber 18 is provided. A high pressure back-upconduit 26 communicates with the lower portion 25 of the connectingchamber 18.

The clamp 7 (not shown in FIGS. 2A-2H) is adapted and positioned forholding a tubular 1 projecting from above into the upper portion 24 ofthe connecting chamber 18 and can take up both axial and rotationalloads. To hold the tubing string 2, a second clamp 27 for retaining atubular projecting from below into the lower portion 25 of theconnecting chamber 18 is provided. For holding the tube string 2 whilethe second clamp 27 is released to allow it to be moved axially alongthe tube string 2, a further clamp (not shown) below the clamp 27 isprovided which is vertically movable as well to allow continuous axialmovement of the tube string 2 while the second clamp 27 is reversed toits initial position.

The connector 9 is suspended in a special bearing 28 which allows theconnector to move axially relative to the main high pressure conduit 13between an upper extreme position and a lower extreme position definedby a flange 61. The downstream end of the main high pressure conduit 13forms a pressure chamber 51 in which the upper end of the connector 9 isaxially displaceable. If the connector 9 is axially displaced inwardly,the volume of fluid in the pressure chamber 51 is decreased. If the mainhigh pressure conduit 13, and thereby the pressure chamber 51, is underpressure, the connector 9 is pushed outward towards the connectingchamber 18 with a force which is proportional to the pressure in thepressure chamber 51—at least as long as the connector 9 is in a positionbetween its extreme upper and lower positions.

If the top end connecting unit 4 moves vertically in response to axialforces exerted onto the connector 9, such movements can be cancelled outby fluid displacement into and out of the chamber 51, the prevailingpressure determining the axial force exerted onto the connector 9, sothat substantial changes of the force exerted onto the connector 9 areavoided. Thus, in as far as the connector 9 is in a position between itsextreme upper and lower positions, the force exerted downward onto theconnector is essentially independent of the position of the top endconnecting unit 4.

Thus, in effect, the pressure in pressure chamber 51 urges the connector9, of which an upper portion forms a pressure transfer memberdisplaceable in the pressure chamber 51, in an axial direction of thetube string 2 pressing the lower end portion of the connector 9 engagingthe tubular 1, in an axial direction of the tube string 2 and towardsthe connecting shell 17.

To achieve that the force with which the tubular 1 is pressed down bythe pressure in the pressure chamber 51 closely matches the force withwhich tubular 1 is pressed upwards by the pressure in the connectingchamber 18, the volume decrease of fluid in the pressure chamber 51 inresponse to inward displacement of the tubular 1 is essentiallyidentical to the simultaneous volume increase of fluid in the connectingchamber 18 of the connecting shell 17. The fluid displacement per unitof axial movement of the tubular 1 determines the force with which thetubular 1 is pressed outward at a given pressure in the pressure chamber51 and the same applies to the connecting chamber 18. Furthermore, thepressures in the pressure chamber 51 and in the connecting chamber 18are substantially equal, so that the forces exerted by the pressure inthe upper portion 24 of the connecting chamber 18 are in principlecancelled out by the forces exerted by the pressure in the pressurechamber 51.

However, the fluid displacement in the upper portion 24 of theconnecting chamber can be selected to be slightly larger than thesimultaneous fluid displacement in the pressure chamber to provide atleast a certain extent of compensation for the weight of for instancethe tubular 1, top end unit 4, of the table 6 and of the clamp 7.

To ensure that the pressure in the pressure chamber 51 is indeedsubstantially equal to the pressure in the connecting chamber 18 when atubular is being stabbed or disconnected, the pressure chamber 51 isarranged to communicate with the connecting chamber 18, when inoperating condition. In the present example this communication isobtained via the tubular 1 that is being stabbed or disconnected and, ifthe preventer 23 is open, via the high pressure conduits 14, 26.

Although it would be possible to provide that one of the seals or bothseals sealing the passages of the upper portion of the connectingchamber and the pressure chamber can move axially with the tubular andthe connector, in the present example, a particularly efficientconstruction is achieved by providing that the seals 16, 21 arestationary relative to the pressure chamber 51 and, respectively, to theconnecting shell 17 and that, in operating condition, the areassurrounded by the seal 21 for sealing off the upper passage 19 of theconnecting shell 17 and by the seal 16 at an end of the main highpressure conduit 13 are essentially identical.

If it is desired to use a top end connecting unit with a particularconnector for adding and/or removing tubulars having differentcross-sectional areas at the seal sealing the upper passage 19 of theconnecting shell 17, the cross-sectional area of the connector at theseal 16 of the pressure chamber 51 is preferably about equal to theaverage cross-sectional area of the different tubulars in the area whichis at the seal sealing the upper passage 19 when a connection is made-upor being broken out.

If the area surrounded by the seal 21 of the passage 19 is slightlylarger than the area surrounded by the seal 16 sealing to the connector9 the upward forces are slightly larger than the compensating forces,which is favourable to taking into account the weight of inter alia thetubular 1, of the top end unit 4, of the table 6 and of the clamp 7.

Usually, the tubulars of a tubing string have an increased thickness atthe coupling ends. As the tubing string 2 is lowered into the well orpulled out of the well, thse portions having an increased thickness passthe seals 21, 22 in the passages 19, 20 of the connecting shell 17.Furthermore, it is advantageous if the opening through the seal 21 ofthe passage 19 is wide when a tubular is to be inserted into the passage19. Therefore, the seals 21, 22 for sealing off the passages 19, 20 ofthe connecting shell 17 are expandable from a receiving condition forallowing insertion of a tubular into the connecting chamber 18 to anexpanded condition for sealing off the opening against a tubular axiallyprojecting into the connecting chamber 18. In a retracted condition, theseals 21, 22 do not need to perform a sealing function against highpressure in the connecting chamber. Expandable seals as described areknown as such and conventionally used for example for sealing off a borehole during underbalanced drilling.

In each of the high pressure conduits 13, 14 and 26 valves 29, 30 forclosing off the high pressure conduits 13, 14 and 26 are provided. Thevalves 29, 30 are each bypassed by a bypass 31, 32 communicating withthe respective one of the high pressure conduits 13, 14 and 26. In eachof the bypasses 31, 32 a bypass-valve 33, 34 (see FIG. 1) is provided.Furthermore, a low pressure conduit 35 communicates with the upperportion 24 of the connecting chamber 18. In this low pressure conduit 35a valve 36 (see FIG. 1) is provided for closing of the low pressureconduit 35.

The valves 33, 34 in the bypasses 31, 32 are substantially smaller thanthe valves 29 and 30 in the high pressure conduits 13, 14 and 26. Ofthese valves only the valves 33, 34 in the bypasses 31, 32 are capableof being operated while a full operating pressure drop over therespective valve exists or is formed as the valve is closed. This allowsto use large valves 29, 30 in the high pressure conduits 13, 14 and 26which can be opened and closed only while no substantial pressure dropover the valve exists or is caused to exist. Such valves aresubstantially less expensive than large valves which can be operated ata full operating pressure drop over the valve. The bypasses 31, 32 allowto even out pressure differences before or while the valves 29, 30 inthe high pressure conduits 13, 14 and 26 are operated. However, to filland empty the connecting chamber 18 or at least the upper portion 24thereof via the bypasses would take a long time.

This problem is avoided by filling, or emptying as the case may be, theconnecting chamber 18 or at least its upper portion 24 via the lowpressure conduit 35 which is of a substantially larger internalcross-section than the bypasses 31, 32. Accordingly, the bypasses 31, 32only serve for topping up the connecting chamber up to the pressure inthe respective high pressure conduit 13, 14 and 26.

Thus, filling up and emptying the connecting chamber up to a pressurewhich is generally in the order of magnitude of 350 bar—typically about250-500 bar—, can be achieved very quickly without resorting to the useof expensive valves in the high pressure lines 13, 14 and 26 which arecapable of being opened or closed while loaded by a pressure drop overthe valve. Furthermore, opening and closing the valve in unloadedcondition only provides the advantage that wear of the valves issubstantially reduced.

It is observed that, while these features are particularly advantageousin combination with a pressure chamber for compensating axial forcescaused by the pressure in the connecting chamber 18, because this allowsto change the pressure in the connecting chamber particularly quicklywithout substantially disturbing the control of stabbing or retractionforces, the use of separate low pressure conduits and the featuresrelated thereto can also be used with advantage in an apparatus in whichno pressure chamber and no associated pressure transfer members areprovided for generating axial forces compensating the forces exerted bythe pressure in the connecting chamber.

In order to allow emptying of the lower portion 25 of the connectingchamber 18 as well, a low pressure conduit 37 communicating with thelower portion 25 of the connecting chamber 18 of the connecting shell 17is provided. This low pressure conduit 37 is provided with a valve 38for closing off that conduit 37.

In combination with pumps 39, 40 (FIG. 2M), the valves 36, 38 in the lowpressure lines 35, 37 form a flow control structure for controlling theflow through the low pressure lines 35, 37. Furthermore, a controlsystem 41 (FIG. 2M) is provided which is operatively connected to thevalves 29, 30 in the high pressure conduits 13, 14, 26, to the bypassvalves 33, 34 and to the flow control structure 36, 38, 39, 40. Thecontrol system 41 is programmed for each time controlling the flowcontrol structure 36, 38, 39, 40 to fill up at least the upper portion24 of the connecting chamber 18 in the connecting shell 17 via the lowpressure conduit 35 before opening of the bypass valve 33. Thus, it isensured that each time only a limited volume of fluid has to pass thebypass 31 to build up the required pressure in the upper portion 24 ofthe connecting chamber 18.

The operation of the shown apparatus is described with reference toFIGS. 2A-2M which show one complete cycle of adding a tubular 1 to atubing string 2.

Before starting a cycle of connecting a tubular 1 to a tubing string 2,the apparatus is operating in a condition in which the valve 30 isclosed and the valve 29 is opened so that fluid (in this example mud)passes via the hose 14, the pressure chamber 51 and the passage 15 inthe connector 9 to the tube string 2, as is represented by arrows 42,43. The connector 9 and the tubing string 2 are lowered into the upperpassage 19 of the connecting shell 17 until a position at leastpartially within the connecting chamber 18 of the connecting shell 17 isreached. This position, in which moreover the upper end of the tubingstring 2 is located in the lower portion 25 of the connecting chamber18, is shown in FIG. 2A. As is further shown in FIG. 2A, the seals 21,22 of the passages 19, 20 are in a non-sealing, retracted condition andthe preventer 23 is open.

Then, the seals 21, 22 of the passages 19, 20 are expanded into asealing, expanded condition and the lower clamp 27 is closed to engagethe tubing string so that the situation shown in FIG. 2B is obtained.

Subsequently, the connecting chamber 18 is filled with mud via the lowpressure conduits 35, 37 by opening the valves 36, 38 as is representedin FIG. 2C by arrows 44, 45. After the connecting chamber 18 has beenfilled or has almost been filled, the valves 36, 38 are closed again toprevent high pressure from reaching the low pressure conduits, and thebypass valve 34 is opened so that the pressure in the connecting chamber18 rises to the pressure of typically 250-500 bar prevailing in the highpressure conduits 13, 14 and 26 (arrow 46). After the pressure drop overthe valve 30 in the back-up high pressure conduit 26 has been equalised,the valve 30 is opened.

In-between, the flow of fluid through the conduit 13 and the tubingstring 2 still continues as is represented by the arrows 42, 43. Then,the connector 9 is disconnected from the tubing string 2 as is shown inFIG. 2D. Since the pressure in the connecting chamber 18 is essentiallyequal to the pressure in the passage 15 through the connector 9 and thepressure in-hue tubing string 2 near its upper end, the flow stillcontinues, now via the connecting chamber 18. A portion of the flow viathe main high pressure conduit 13 may be replaced by through the back-uphigh pressure conduit 26. This portion becomes substantial at least whenthe connector 9 is moved further up and away from the tubing string 2until a position above the level of the preventer 23, as is representedin FIG. 2E by the reduced length of the arrows 42′, 43′ and the arrow46.

Next, as shown in FIG. 2F, the preventer 23 is closed, so that the flowthrough these conduits is completely replaced by a flow through theback-up high pressure conduit 26 (arrow 46).

After the preventer 23 has been closed, the valves 29, 33 in the highpressure line 14 connecting to the main high pressure conduit 13 areclosed and the upper connecting chamber portion 24 drained via the lowpressure conduit 35 communicating therewith by opening the valve 36 andactivating the pump 33 (arrow 47).

Then, the seal 21 in the upper passage 19 is retracted to facilitatewithdrawal of the connector 9 from the connecting chamber 15 and theconnector 9 is withdrawn from the connecting chamber 18 as is shown inFIG. 2H. In the meantime, the flow of fluid through the tubing string 2is maintained via the back-up high pressure conduit 26 and the lowerportion 25 of the connecting chamber 18 under the preventer 23 (arrow46).

In FIG. 2I a next stage of the present method is shown in which atubular 1 to be added to the tubing string 2 has been connected to theconnector 9 and is being lowered into the connecting chamber 18. Tofurther speed up the method, it is also possible to make use of twoconnectors or two top end connecting unit-which alternate each other sothat each time a connector with a tubular connected thereto is directlyavailable and no time is lost with connecting a tubular to a connectorin the time between lifting the connector out of the connecting chamberand bringing the new tubular in line with tubing string 2.

Once the coupling portion 12 at the lower end of the new tubular 1 iscompletely within the connecting chamber 18, the seal 21 in the upperpassage 19 of the connecting shell 17 is expanded to seal against thestem of the new tubular 1 (FIG. 2J). As is also shown in FIG. 2J, theupper portion 24 of the connecting chamber 18 is filled again via thelow pressure conduit 35 communicating therewith by activating the pump39 (arrow 48). The valve 36 has been left open since the draining of theupper portion 24 of the connecting chamber 18. It is observed that theends of the low pressure conduit 35, 37 remote from the connectingchamber 18 need only communicate with a reservoir of sufficient size totemporarily store fluid drained from the connecting chamber 18. If sucha reservoir is positioned at a suitable level, the pumps 39, 40 needonly be operated for either draining or filling the connecting chamber,the opposite flow being obtainable by simply letting the fluid flow backdown. After the connecting chamber 18 has been filled sufficiently, thevalve 36 is closed.

Then, the main high pressure conduit 13 (or another one of the highpressure conduits if a different connector connected to a different highpressure branch is used) communicating with the new tubular 1 and theupper portion 24 of the connecting chamber 18 is brought under pressureby opening the valve 33 in the bypass 31 around the valve 29 in the highpressure hose 14 in-line with the main high pressure conduit 13 (arrow49). As was discussed, the upward force exerted onto the new tubular 1by the pressure in the connecting chamber 18 is compensated by thepressure in the pressure chamber 51 pushing the connector 9, and therebythe tubular 1 connected thereto, downward.

Then, as is shown in FIG. 2K, the preventer 23 and the valve 29 in thehigh pressure hose 14 in-line with the main high pressure conduit 13 areopened. Since the pressures on opposite sides of the preventer 23 areequalised each time before the preventer 23 is opened, the highoperating pressure is applied to the preventer only while the preventeris closed and not while it is being opened or closed. Therefore, thepreventer can be of a relatively simple design and wear of the preventeris reduced.

Thus, a portion of the fluid flow is again lead via the high pressurehose 14 and the main high pressure conduit 13 (arrows 42′, 43′) and theflow via the high pressure back-up conduit 26 is reduced accordingly(arrow 46′).

Subsequently, the top end connecting unit 4 is lowered until the lowerend of the new tubular 1 is closely above the upper end of the tubingstring 2 (FIG. 1). In this position, the tubular 1 is clamped by theclamp 7 and slightly lifted from its lowest position relative to the topend connecting unit 4 by lowering the top end connecting unit slightlyfurther. Axial movement of the new tubular 1 is now controlled by theaxial movement of the clamp 7, the top end connecting unit 4 merelyfollowing such movements to keep the connector 9 from reaching itsextreme upper and lower positions relative to the top end connectingunit. The clamp 7 accurately controls the axial stabbing force appliedto the tubular 1 as it is lowered and engages the upper end of thetubing string 2 (FIG. 2L). The axial forces which have to be exerted bythe clamp 7 and by the cylinders 60 for moving the table 6 carrying theclamp 7 are limited because the axial forces exerted by the pressure inthe connecting chamber 18 and in the main high pressure conduit 13essentially cancel each other out, even if these pressures wouldfluctuate erratically (any substantial pressure differences beingprevented by communication between the connecting chamber 18 and themain high pressure conduit 13 via the tubular 1).

Once the connection between the tubular 1 and the tubing string 2 hasbeen made up, generally by also twisting the tubular 1 relative to thetubing string 2 which preferably has continued to rotate and to belowered, the fluid flow, which has not been interrupted, again runsentirely via the main high pressure conduit 13 (arrows 42, 43) and thevalve 30 in the high pressure back-up conduit 26 as well as its bypassvalve 34 are closed. Then, the connecting chamber 18 is drained byopening the valve 38 in the low pressure conduit connected to the lowerportion of the connecting chamber 18 (arrow 50).

The clamps 7, are released and after the connecting chamber 18 has beendrained, the seals 21, 22 in the passages 19, 20 in the connecting shell17 are retracted again to allow continuation of the lowering of thetubing string 2.

Since the fluid is removed from the connecting chamber 18 after make-upof the connection and before the connection leaves the connectingchamber 18, the lower seal need not fulfil any sealing function whilethe tubing string 2 is being lowered and portions of different thicknesspass the lower passage 20 in the connecting shell 17.

However, especially if the tubing string is reasonably smooth on itsoutside, it is preferred to leave fluid in the connecting chamber 18 andto pass the connection between successive tubulars through the lowerseal 22 of the connecting shell while this seal seals against the tubingstring 2. Thus, the need of draining and refilling the connectingchamber 18 is obviated. Seals for sealing against pipe sections andconnections having a different diameter, both axially passing through,are commercially available.

In FIG. 3 a presently most preferred embodiment of the present inventionis shown. In as far as the design shown in FIG. 3 essentiallycorresponds to the design shown in FIG. 1, mutually identical referencenumerals are used.

In the apparatus shown in FIG. 3, the first engagement structure isformed by a clamp 107 adapted for transmitting both axial forces and atorque to the tubular 1. Accordingly, the main high pressure conduit 113is of a different design without a pressure chamber into which theconnector can move axially. The top end connecting unit 104, throughwhich the main high pressure conduit 113 extends, is suspended from ahoist 103. To allow tube sections to be connected to or disconnectedfrom the fluid circuit while fluid is being circulated through the topend of the tube string 2, preferably two or more top end connectingunits 104 and a device for laterally moving the top end connecting units104 are provided as is described in international patent applicationPCT/NL97/00726, which corresponds to U.S. Pat. No. 6,435,280 and whichissued to Van Wechem et al. on Aug. 20, 2002 and which is incorporatedherein by reference.

Pressure transfer members in the form of pistons 152 and a carrier table153 are connected to the clamp 107 for transferring forces compensatingthe forces exerted by the pressure in the connecting chamber 18 in aconnecting shell 117 to the clamp 107.

The clamp 107 is located for engaging the tubular 1 between theconnecting chamber 18 and the top end connecting unit 104.

In operation, the first engagement structure engages the tubular betweenthe connecting shell 117 and the top end connecting unit 4. Thisprovides the advantage that the tubular 1 is substantially lesssusceptible to buckling under the axial compression loads exertedthereto by the pressures in the connecting chamber 18 and by the clamp107. This is of particular advantage when each time not a singletubular, but a subassembly of two or more tubulars is added to thestring 2.

In the present apparatus the likelihood of buckling is practicallynon-existent because the clamp 107 is located for engaging the tubularclosely adjacent the connecting chamber 18. The drives for drivingrotation of the clamp 107 are not shown in FIG. 3. For these features,reference is made to international patent application PCT/NL 97/00727.

In this example, two pressure chambers 151 are formed by cylinderchambers 151 in pressure cylinders 154. Since the pressure cylinders 154are directly connected to the connecting shell 117, the reactive forcescompensating the upward forces generated by the pressure in theconnecting chamber 18 need not be transferred via a trajectory involvingother parts of the apparatus.

The pressure in the pressure chambers 151, which have venting openings157 below the pistons 155, urges the pistons 152 displaceable in thepressure chambers 151 in an axial direction of the tube string 2pressing the clamp 107 engaging the tubular 1 in an axial direction ofthe tube string 2 towards the connecting shell 117 and therebycompensates the forces exerted by the pressure in the connecting chamber18.

To control the axial displacements of the tubular 1 and the stabbing andretraction forces, a separate set of operating cylinders 155 isprovided, which are mounted between the carrier table 153 and theconnecting shell 117 as well. The housings of the cylinders 155 are notmounted to the seals 21, but, as seen in the present representation,behind the seals to the housing of the connecting shell 117. A table 106supports the carrier table 153 carrying the clamp 107 and the connectingshell relative to the suspension structure 5.

In principle, the pressure in the pressure chambers 151 could bedifferent form the pressure in the connecting chamber 18, for instanceby being in a proportional relation therewith. In the present apparatus,the pressure chambers 151 are arranged to communicate with theconnecting chamber 18 via conduits 156 directly connecting the pressurechambers 151 with the upper portion 24 of the connecting chamber 18.Thus, it is ensured in a particularly simple manner that the pressure inthe pressure chambers 151 is substantially equal to the pressure in atleast the upper portion 24 of the connecting chamber 18.

It is evident to the skilled person that many modifications can be madeto the example shown above without departing from the scope of theinvention. For instance, separate clamps can be used for taking uprotational and axial loads, the clamps or alternative clamps can belocated within the connecting shell instead of outside the connectingshell, pressure differences over the main valves can be equalised byopening the valves in a special manner or by opening special ports inthe valve bodies, and the seal at the end of the main high pressureconduit can be positioned to seal against the stem of a tubular.

What is claimed is:
 1. An apparatus for connecting and disconnectingtubulars and a tubing string suspended in a well and for axiallydisplacing that tubing string, comprising: a suspension structure; a topend connecting unit including at least one connector for connection to atubular axially projecting from said at least one connector, and atleast one main high pressure conduit communicating with said at leastone connector for providing circulation through a tubular connected tosaid at least one connector; a connecting shell bounding a connectingchamber, said connecting shell having an upper passage and a lowerpassage for receiving tubulars, means for sealing off said passagesagainst tubulars extending through said passages, a preventer forseparating an upper portion of said connecting chamber from a lowerportion of said connecting chamber, and a back-up high pressure conduitcommunicating with said lower portion of said connecting chamber; afirst engagement structure for engaging a tubular projecting from aboveinto said upper portion of said connecting chamber, said firstengagement structure being carried by said suspension structure; and asecond engagement structure for engaging a tubular projecting from belowinto said lower portion of said connecting chamber; wherein at leastsaid top end connecting unit is movable up and down relative to saidsuspension structure; and said connecting shell is located below saidtop end connecting unit; characterized by a pressure compensatingstructure for compensating axial force exerted by pressure in said upperportion of said connecting chamber pressing a tubular projecting fromsaid upper portion of said connecting chamber in an axial direction outof said connecting chamber, said pressure compensating structureincluding at least one pressure chamber and at least one pressuretransfer member formed by the connector or by a separate pressuretransfer member connectable to a tubular; said pressure chamber having apassage and means for sealing off said passage against said at least onepressure transfer member, said at least one pressure chamber beingconnected to said connecting chamber for maintaining a pressure in saidat least one pressure chamber corresponding to pressure in at least saidupper portion of said connecting chamber, and said at least one pressuretransfer member being displaceable in said at least one pressure chamberand arranged for transferring a force axially pressing the tubular intosaid connecting chamber in reaction to pressure in said at least onepressure chamber.
 2. An apparatus according to claim 1, wherein said atleast one pressure chamber is arranged to communicate with saidconnecting chamber, when in operating condition.
 3. An apparatusaccording to claim 1, wherein said at least one pressure transfer memberis connected to said first engagement structure, said first engagementstructure being located for engaging said tubular between saidconnecting chamber and said top end connecting unit.
 4. An apparatusaccording to claim 1, wherein said at least one pressure transfer memberis connected to said first engagement structure, said first engagementstructure being located for engaging said tubular closely adjacent saidconnecting chamber.
 5. An apparatus according to claim 1, wherein saidat least one pressure chamber has a housing directly connected to saidconnecting shell.
 6. An apparatus according to claim 1, wherein, inoperating condition, the displacement of fluid in said at least onepressure chamber in response to a tubular projecting from said at leastone connector into said connecting chamber being axially displacedrelative to said connecting chamber and to said at least one pressurechamber is of essentially the same volume as the simultaneousdisplacement of fluid in said connecting chamber caused by said movementof said tubular.
 7. An apparatus according to claim 6, wherein, inoperating condition, the displacement of fluid in said at least onepressure chamber in response to a tubular projecting from said at leastone connector into said connecting chamber being axially displacedrelative to said connecting chamber and to said at least one pressurechamber is smaller than the simultaneous displacement of fluid in saidconnecting chamber caused by said movement of said tubular, thedifference between said fluid displacements being adapted to compensatefor weight of said tubular and of equipment being displaced therewith.8. An apparatus according to claim 1, wherein, in operating condition,said means for sealing off said upper passage of said connecting shellsurround an area urged in axial direction by said fluid pressure, andsaid means for sealing off said passage of said at least one pressurechamber surround at least one other area urged in axial direction bysaid fluid pressure, said area and said at least one other area eachhaving an aggregated size in axial projection, said aggregated sizesbeing essentially identical to each other.
 9. An apparatus according toclaim 8, wherein, in operating condition, the size of said areasurrounded by said means for sealing off said upper passage of saidconnecting shell is larger than the aggregated size of said at least onearea surrounded by said means for sealing off said passage of said atleast one pressure chamber, the difference between said sizes beingadapted to compensate for weight of said tubular and equipment beingdisplaced therewith.
 10. An apparatus according to claim 1, wherein saidseals for sealing off said passages of said connecting shell areexpandable from a receiving condition for allowing insertion of atubular into said connecting chamber into an expanded condition forsealing off said opening against a tubular axially projecting into saidconnecting chamber.
 11. An apparatus according to claim 1, furthercomprising a valve in each of said high pressure conduits for closingoff said high pressure conduits, bypasses communicating with each one ofsaid high pressure conduits and bypassing said respective valves in saidhigh pressure conduits, a bypass-valve in each of said bypasses, and alow pressure conduit communicating with said upper portion of saidconnecting chamber.
 12. An apparatus according to claim 11, furthercomprising a low pressure conduit communicating with said lower portionof said connecting chamber of said connecting shell.
 13. An apparatusaccording to claim 11, further comprising a flow control structure forcontrolling the flow through said low pressure conduit and a controlsystem operatively connected to said valves in said high pressureconduits communicating with said at least one connector and with saidlower portion of said connecting chamber of said connecting shell, tosaid valves in said bypass conduits and to said flow control structure,said control system being programmed for each time controlling said flowcontrol structure to fill up at least said upper portion of saidconnecting chamber in said connecting shell via said low pressureconduit before opening at least one of said bypass valves.
 14. A methodof assembling a tubing string projecting into a well, including thesteps of: providing a top end connecting unit including at least oneconnector for connection to a tubular axially projecting from said atleast one connector, and at least one main high pressure conduitcommunicating with said at least one connector for allowing circulationthrough a tubular connected to said at least one connector; providing aconnecting shell below said top end connecting unit, said shell boundinga connecting chamber and having an upper passage and a lower passagecoaxial therewith for receiving tubulars, seals for sealing off saidpassages against tubulars extending through said passages, a preventerfor separating an upper portion of said connecting chamber from a lowerportion of said connecting chamber, and a back-up high pressure conduitcommunicating with said lower portion of said connecting chamber;lowering said at least one connector and a tubing string connectedthereto into said connecting shell until a position at least partiallywithin said connecting chamber of said connecting shell; disconnectingsaid at least one connector from said tubing string; lifting said atleast one connector into a position in which a lowest portion thereof islocated above said preventer; closing off said preventer; removingfeeding pressure from said at least one main high pressure conduit andfrom said upper portion of said connecting chamber and withdrawing saidat least one connector from said connecting chamber; connecting atubular to one of said connectors and lowering said tubular into saidupper passage; bringing one of said at least one main high pressureconduits in communicating with said tubular and said upper portion ofsaid connecting chamber under pressure and opening said preventer; andfurther lowering said tubular and making up a connection between saidtubular and said tubing string; characterized in that, after said stepof lowering said tubular into said upper passage, pressure in at leastone pressure chamber is maintained at a level corresponding to pressurein said upper portion connecting chamber, axially urging said tubulartowards said connecting chamber and at least partially balancing outupward pressure exerted to said tubular towards said connecting chamberand at least partially balancing out upward pressure exerted to saidtubular by the pressure applied to said upper portion of said connectingchamber.
 15. A method according to claim 14, wherein said pressure insaid at least one pressure chamber urges at least one pressure transfermember displaceable in said at least one pressure chamber in an axialdirection of said tube string pressing said tubular in an axialdirection of said tubing string towards said connecting shell.
 16. Amethod according to claim 15, wherein axial forces in the direction ofsaid tubing string are exerted onto said tubular in a position betweensaid connecting shell and said top end connecting unit.
 17. A methodaccording to claim 15, wherein axial forces in the direction of saidtubing string are exerted onto said tubular in a position closelyadjacent said connecting shell.
 18. A method according to claim 14,wherein axial displacement of said tubular relative to said at least onepressure chamber and relative to said upper portion of said connectingchamber is associated to a fluid displacement in said upper portion ofsaid connecting chamber and a fluid displacement of essentially the samevolume in said at least one pressure chamber.
 19. A method according toclaim 18, wherein the fluid displacement in said upper portion of saidconnecting chamber associated to axial displacement of said tubularrelative to said at least one pressure chamber and relative to saidupper portion of said connection chamber is larger than the simultaneousfluid displacement in said at least one pressure chamber, the differencebetween said displacements being adapted to compensate for weight ofsaid tubular and equipment being displaced therewith.
 20. A methodaccording to claim 14, wherein said pressure in said at least onepressure chamber is controlled by communication with said upper portionof said chamber in said connecting shell.
 21. A method according toclaim 14, wherein pressures on opposite sides of said preventer areequalized before said preventer is opened.
 22. A method according toclaim 14, wherein fluid is left in said connecting chamber after themaking or breaking of said connection and the lower passage sealsagainst said tubing string as connections pass through said lowerpassage.
 23. A method according to claim 14, wherein, each time at leastsaid upper portion of said connecting chamber is filled, fluid istransferred into said connecting chamber via a low pressure conduit,subsequently said low pressure conduit is closed off and subsequentlysaid connecting chamber is brought under a higher operating pressure byopening a valve in a bypass of said at least one main high pressureconduit communicating with said connecting chamber.
 24. A methodaccording to claim 23, wherein each time fluid is removed from saidconnecting chamber, valves of high pressure conduits communicating withsaid connecting chamber are closed, subsequently a low pressure conduitcommunicating with said connecting chamber is opened and fluid in saidconnecting chamber is removed via said low pressure conduit.
 25. Amethod of disassembling a tubing string projecting into a well,including the steps of: providing a top end connecting unit including atleast one connector for connection to a tubular axially projecting fromsaid at least one connector for allowing circulation through a tubularconnected to said at least one connector; providing a connecting shellbelow said top end connecting unit, said shell bounding a connectingchamber and having an upper passage and a lower passage coaxialtherewith for receiving tubulars, seals for sealing off said passagesagainst tubulars extending through said passages, a preventer forseparating an upper portion of said connecting chamber from a lowerportion of said connecting chamber, and a back-up high pressure conduitcommunicating with said lower portion of said connecting chamber;lowering said at least one connector into said connecting shell until aposition at least partially within said connecting chamber of saidconnecting shell; bringing one of said at least one main high pressureconduit in communication with said connector under pressure and openingsaid preventer; further lowering said connector and making up aconnection between said tubular and said tubing string; connecting saidat least one connector to said tubing string (2); lifting said at leastone connector and a tubular connected thereto into a position in which alowest portion of said tubular is located in said lower portion of saidconnecting chamber; breaking up a connection between said tubular andsaid tubing string; lifting said at least one connector and a tubularconnected thereto into a position in which a lowest portion of saidtubular is located in said upper portion of said connecting chamber andabove said preventer; closing off said preventer; removing feedingpressure from said at least one main high pressure conduit and from saidupper portion of said connecting chamber and withdrawing said tubularfrom said connecting chamber; characterized in that, after said step oflowering said at least one connector into said connecting shell,pressure in at least one pressure chamber is maintained at a levelcorresponding to pressure in said upper portion of said connectingchamber, axially urging said connector towards said connecting chamberand at least partially balancing out upward pressure exerted to saidconnector by the pressure applied to said upper portion of saidconnecting chamber.